1. Field of the Invention
The present invention relates to a drive circuit of an indicator that emits a light by supplying a d.c. current, and to a light emitting indicator drive circuit that conducts a correcting process for reducing a difference in the luminance of emitted light of a plurality of display elements each having a different area.
2. Description of the Related Art
FIG. 5 shows a conventional light emitting indicator drive circuit. Also, FIG. 6 is a plan view showing the element shape of a conventional light emitting indicator. Display elements 501 to 520 showed in FIG. 5 are connected to each other in the form of a matrix. An output of a constant-current source 521 having a switching function and allowing a current to flow out is connected to the plus electrode sides of each of the display elements 501, 506, 511 and 516. Likewise, an output of a constant-current source 522 is connected to the display elements 502, 507, 512 and 517, an output of a constant-current source 523 is connected to the display elements 503, 508, 513 and 518, an output of a constant-current source 524 is connected to the display elements 804, 509, 514 and 519, and an output of a constant-current source 525 is connected to the display elements 505, 510, 515 and 520, respectively. On the other hand, a switch 526 that allows a current to flow into the ground is connected to the minus electrode sides of the display elements 501, 502, 503, 504 and 505, respectively. a switch 527 is connected to the display elements 506, 507, 508, 509 and 510, a switch 528 is connected to the display elements 511, 512, 513, 514 and 515, and a switch 529 is connected to the display elements 516, 517, 518, 519 and 520, respectively.
As shown in FIG. 6, in the conventional example, the areas of the display elements disposed within the light emitting indicator 61 are identical with each other, and the constant current sources 521 to 525 supply the same current. For that reason, the current densities of all the display elements become equal to each other, to thereby obtain substantially the same luminance.
In order to turn on/off the respective display elements, each of the constant current sources 521 to 525 has a switching function so that the respective constant current sources can be turned on/off. On the other hand, the switches 526 to 529 are sequentially turned on one by one in a time sharing manner, and there is no case where two or more switches are turned on at the same time. In the case of turning on the display element 501, the constant current source 521 and the switch 526 are turned on. Similarly, all the display elements can be selectively turned on by the combination of the constant current sources 521 to 525 with the switches 526 to 529.
With the above structure, a very large number of light emitting elements are arranged to conduct dot matrix display, thereby being capable of executing various display.
If the dot matrix display is applied to an indicator such as a wristwatch which requires a small size and a low power, the power consumption of the driver and the chip size become large, and also if no fine dot matrix display is used, fine character display cannot be made. In this case, segment display is advantageous, but it is difficult to make the areas of the respective segments identical with each other, as a result of which if the segments are driven by a constant current source having the same current value, the density of current depends on the area of the respective segments to produce a difference in luminance between the respective segments.
In particular, even a difference of several percent in luminance between the adjacent segments is conspicuous. For that reason, if the luminance between the respective segments is not corrected, the display quality is remarkably degraded so that the dot matrix display cannot be applied to a wristwatch that requires high fashionability, or the like.